Controlling Insects in Stored Grain Using Modified Atmospheres of Elevated Carbon Dioxide.
On an annual average, Canada produces about 55 Mt of grains, worth about 6 billion dollars. Thus, grains are an important contributor to the Canadian economy and any losses which occur in storage at any point including transport and marketing directly reduce farm income.
The rusty grain beetle is the most economically important insect pest of stored grain in Canada. It is an endemic pest on farms in Western Canada. Canadian grain is sold on the basis of a zero tolerance for live insects. Modified atmospheres with elevated [CO.sub.2] levels can be an effective means of preventing or killing pests in stored products. Most liquid-formulated fumigants such as ethylene dibromide which were used to kill insects in stored products, have been banned because of a possible risk of carcinogenicity in mammals. Two fumigants remain: methyl bromide, which may be banned in the USA and Canada during a re-certification process to be completed in the near future because it has been identified as an agent for the depletion of the ozone layer, and phosphine, which is widely used throughout the world but is at risk because insects are developing resistance to it. Malathion, the only contact insecticide registered in Canada, has been recommended by the Environment Protection Agency (EPA) for de-r egistration in the USA and soon it might be de-registered in Canada. Modified atmospheres are superior to fumigants in that they are less toxic to humans, and are preferable to contact insecticides because no chemical residues are left on the stored product.
To maintain Canada's reputation of exporting high-quality wheat free of insects and chemical residues and in response to increased political and consumer demands for improved standards of health, safety, and environmental quality, research on controlling insects using physical methods is needed. Our major competitors are spending considerable financial resources to develop new, nonchemical methods for keeping their grain relatively insect-free.
The main objective of the proposed project was to develop strategies to control rusty grain beetles, in grains stored 1. on farms in welded-steel bins, 2. at a primary elevator in a concrete bin, and 3. in a hopper railcar by exposing the stored grain to modified atmospheres (MA) of elevated carbon dioxide ([CO.sub.2]).
Definition of Modified Atmospheres (MA)
Disinfestation of stored grain using MA involves the alteration of the composition of the intergranular storage gases ([CO.sub.2], oxygen ([O.sub.2]), and nitrogen ([N.sub.2])), to render the atmosphere in the stores lethal to pests. The MA does not include an alteration of the storage atmosphere by addition of toxic gases such as phosphine or methyl bromide, nor regulation or alteration of the atmospheric water content. The MA may be achieved in several ways: by adding gaseous, liquid, or solid [CO.sub.2]; by adding a gas of low [O.sub.2] content (e.g., pure [N.sub.2] or output from a hydrocarbon burner); or by allowing metabolic processes within a storage to remove [O.sub.2], usually with associated release of [CO.sub.2]. Such atmospheres are referred to as 'high-[CO.sub.2]', 'low-[O.sub.2]', and 'hermetic storage' atmospheres, respectively. They are collectively known as 'modified atmospheres'. The high-[CO.sub.2] atmospheres usually perform better than the low-[O.sub.2] atmospheres or hermetic storage for controlling rusty grain beetles in wheat.
A sealing method was developed to retain and uniformly maintain [CO.sub.2] concentrations in welded steel farm bins. The method is flexible and can be used for any bin opening. Physical exclusion of insects from sealed bins was identified as an important methodology for incorporation into an integrated pest management strategy. See Figures 1 and 2.
A methodology was developed to calculate leakage areas in imperfectly sealed bins from pressure decay times obtained from pressure decay tests. With the predicted leakage area and the planned initial [CO.sub.2] concentration, the [CO.sub.2] concentration profile can be projected over time and the required length of fumigation and amounts of [CO.sub.2] to be added can be calculated. A new concept of cumulative lethality index was developed to predict the mortality of rusty grain beetles in bins with declining [CO.sub.2] concentrations. These relationships, therefore, are important tools for successful fumigation of insects in stored grain using [CO.sub.2].
Adult rusty grain beetles were exposed to elevated levels of [CO.sub.2] in farm-scale welded-steel hopper bins. The insects were placed in escape-proof cages modified from grain probe traps. One hundred insects and 1 g of wheat germ were placed inside each cage. Insect cages were placed at four elevations within the grain mass and at three radial locations for each elevation, except the lowest elevation where only two radial locations were possible because of the shape of the bottom cone of the bin. During the fumigations, grain temperatures, and [CO.sub.2] concentrations were measured and recorded at daily intervals. Following the fumigation, the cages were removed from the bin and insects were identified as being either dead or alive. Observed insect mortalities were in excess of 99 per cent for three experiments of 10 days duration, but were only 95 and 80 per cent for two experiments of 4 days duration.
For the concrete elevator bin, adult rusty grain beetles and red flour beetles were exposed to elevated levels of [CO.sub.2]. Fifty adults of each species were placed into pitfall traps and were suspended from rope at heights of 0, 6, 12, 18, 24, and 30 m from the bottom of the bin. Tests were conducted for durations of 2, 3, 4, and 5 days. The [CO.sub.2] concentrations ranged from 46-94, 9.5-91, and 7.7-89 per cent at the 12, 18, and 24 m heights, respectively. All insects were killed at heights of 0, 6, 12, 18, and 24 m, except in the 2 day test in which one rusty grain beetle survived at each of the 18 and 24 m heights. Mortality was lowest at 30 m, where survival ranged from 72-98 per cent for rusty grain beetles and 94-100 per cent for red flour beetles. Survival at the top was directly attributable to the loss of [CO.sub.2] through the bin hatch which was the greatest at this position.
Caged adult rusty grain beetles and red flour beetles were exposed to elevated [CO.sub.2] concentrations in the railcar, but the resulting [CO.sub.2] concentrations were too low to cause mortality. The insects remained alive in the grain mass for all five experiments that were conducted. Fumigation of railcars with [CO.sub.2] was considered to be a failure unless the railcars can be permanently modified to improve gas-tightness.
Mann, D. D., D.S. Jayas, N.D.G. White, and W.E. Muir, 'Sealing of welded steel hopper bins for carbon dioxide fumigation of stored grain', Canadian Agricultural Engineering, 39:91-97, 1997.
Digvir S. Jayas, is a professor in the department of biosystems engineering and associate dean (research) in the Faculty of Agricultural and Food Sciences at the University of Manitoba, Winnipeg, MB. He is a professional engineer and a professional agrologist. He conducts research in the areas of physical properties of agricultural products; modified atmosphere storage of grains, oilseeds, potatoes, and meat; mathematical modelling of stored grain ecosystems; and digital image processing for grading and processing operations in the agri-food industry.